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Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
loneoceans wrote ...
You can't just make a GDT by winding any number of turns on any core. It depends on a variety of factors - what frequency are you running the coil at, what sort of power etc. There is no rule of thumb for number of turns either, but if you have too many turns, your core will saturate! You would actually want to use the minimum number of turns without saturating the core. This all depends on the characteristics of your toroid material etc. Finally, I'm pretty sure most power supply toroids will not work especially if they are powered iron. You would want a ferrite core with a high permeability.
It's correct to say that you want to use the minimum number of turns that does not saturate the core, or a bit more if some extra leakage inductance can be tolerated. The core does not saturate with excessive number of turns. It saturates with too few turns. Saturation occurs when the magnetic flux density B in the core becomes too high. The maximum B is proportional to the input voltage and inversely proportional to the frequency and to the number of turns. Too few turns also decrease the primary magnetizing inductance, increasing the input current that the driver must provide just to keep the input voltage waveform even without load. Too many turns increase the leakage inductance, or equivalent series inductance of the transformer, and the series resistance too. This is problematic with the high input capacitance of the IGBTs that the GDT drives.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
I should note I use ~14,000AL cores.
Well, the AL value of the cores I posted is nowhere near 14,000, but I am curious if they would work anyway. If I'm lucky I may be able to get some of those cores for free, so I would like to try different options. Another core on the same site is the ZW42508TC (again, link opens PDF datasheet):
It has a permeability of 10,000 and is 9660AL. What would you recommend? Again, this is my first SSTC so I'm not sure what all to look for to find appropriate cores for a GDT.
The first SSTC I made used a 3300AL core and worked at 600khz. After I got a scope and saw the gate waveforms I switched to the high AL cores, which when nothing else was changed resulted in clean gate drive. Just my anecdotal evidence.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
The first SSTC I made used a 3300AL core and worked at 600khz. After I got a scope and saw the gate waveforms I switched to the high AL cores, which when nothing else was changed resulted in clean gate drive. Just my anecdotal evidence.
That makes sense. I think I asked it earlier but nobody responded--is there a favorite source of GDT cores among the Tesla coil builder community? I've checked a variety of websites but have yet to find one that has cores with AL values of 10,000 or greater.
On my site (sigurthrenterprises.com) in the USSTCC data file is a Bill of Materials which contains the digikey part number for the cores I use. I'd get it for you myself but I'm replying using my phone and cannot download the files on it.
It's correct to say that you want to use the minimum number of turns that does not saturate the core, or a bit more if some extra leakage inductance can be tolerated. The core does not saturate with excessive number of turns. It saturates with too few turns. Saturation occurs when the magnetic flux density B in the core becomes too high. The maximum B is proportional to the input voltage and inversely proportional to the frequency and to the number of turns. Too few turns also decrease the primary magnetizing inductance, increasing the input current that the driver must provide just to keep the input voltage waveform even without load. Too many turns increase the leakage inductance, or equivalent series inductance of the transformer, and the series resistance too. This is problematic with the high input capacitance of the IGBTs that the GDT drives.
That's right! I got them mixed up the other way. The flux density is volts per (area * freq * number of turns), so indeed use the minimum number not to saturate. People used to buy electronics goldmine cores for really low prices. I don't think they have them anymore, but I think easternvoltageresearch sells ferrite cores of the 77/78 type material from fair-rite.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
On my site (sigurthrenterprises.com) in the USSTCC data file is a Bill of Materials which contains the digikey part number for the cores I use. I'd get it for you myself but I'm replying using my phone and cannot download the files on it.
Ah, many thanks! Found just what I was looking for, at a price I was expecting. I'm thinking I'll pick up 4 for now, since I'm rapidly closing in on the budget for this coil, but I'll need to remember where to find your data file for the future. I was looking on DigiKey the other day for cores but couldn't seem to find any suitable ones. This is exactly what I was looking for though!
loneoceans wrote ...
People used to buy electronics goldmine cores for really low prices. I don't think they have them anymore, but I think easternvoltageresearch sells ferrite cores of the 77/78 type material from fair-rite.
I checked easternvoltageresearch and the cores I found (that cost less than the ones on digikey) seem to have a low (2000) permeability. I'm thinking I'll just jump on the digikey ones for now.
Thanks all, I'll post back when I have more updates. Regards, Matt
UPDATE: Just thought I'd post the images of my secondary coil and topload. Both are sort of lashed together, at the moment. I wound the primary several years ago when I was building my SGTC. The topload is 3" diameter aluminum dryer duct, bent into a toroid with an OD of around 13" (actually 12.75"). That was the tightest I could bend it without damaging the duct. This gives me a resonant frequency of around 218kHz. I am hoping the topload isn't too large. If it is, I'm sure I could figure something else out. I also plan to fully wrap the entire toroid in aluminum tape to make it look better, and to improve conduction. So I will not be using it half-and-half, as it is now.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi guys,
Another couple of days have gone by and I have a another question:
First, must any special considerations be taken into account for the bridge rectifier supplying power to the H-Bridge? For example, I plan to allow no more than 120VRMS into the bridge. It will be controlled by a Variac. What information will I need to know before choosing a rectifier? I currently have an RBV-1306 that I pulled from an old amplifier. It's datasheet can be found here:
According to the document, this bridge rectifier is designed for 600V and 13A (with a heatsink, which I do have). I'm thinking this should be fine, but I'd like to verify it first before I release its magic smoke
You need to know what peak/RMS current to expect. I get a good approximation by determining the reactance of the primary at the resonant frequency of the secondary. Plug this in to get an approximation of the peak/RMS current. Use the rectified and smoothed DC Bus voltage if you're going to run CW on smoothed DC, but you can use the RMS voltage if you're not going to run CW or aren't using a smoothing cap (or are using small dc blocking caps on a half bridge). Remember that it is just an estimation, but it's always served me well.
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First SSTC design - Need some critique
The flux density is volts per (area * freq * number of turns), so indeed use the minimum number not to saturate. People used to buy electronics goldmine cores for really low prices. I don't think they have them anymore, but I think easternvoltageresearch sells ferrite cores of the 77/78 type material from fair-rite.
